Potassium inwardly rectifying channels (Kir) stabilize the membrane potential and carry K+ ions. Kir are dominant in the astrocytic Muller (glial) cells responsible for maintaining extracellular homeostasis in the retina. Excitation of neurons increases levels of extracellular potassium ions (K+) at synapses, which if uncorrected would result in depolarization of neurons and a loss of synaptic transmission. Kir channels serve to equaliz4e intraretinal K+ gradients by a mechanism called spatial buffering or K" siphoning. Glial cells carry K+ currents inward in regions where the extracellular K+ concentration is increased and outward at more distant regions. In order to extrude K+ in distant regions, Muller cells need either one type of Kir with rectification that can be quickly regulated (to have outward current) or to have express additional Kir channels with weak or little rectification. The only Kir described in Muller cells to date is Kir 4.1. This channel is strongly rectifying and requires ATP to function and, thereby, could not function for K+ siphoning or when ATP is depleted (i.e., anoxia.). Our preliminary data demonstrate the existence of another Kir subunit, Kir 6.1 (KATP) expressed in Muller cells, which may complete the above- mentioned requirements. Our working hypothesis is that there are two possible ways of how these different subunits (Kir4.1 AND Kir6.1) may collaborate in Muller cell functioning, (i) by co-expression in functional heteromers, or (ii) by mutual switching between two distinct homomeric channels. Our long-term goal is to determine the identify and regulation of Kir channels from retinal glial cells. This will be accomplished using a multi-faceted approach. (1) By determining which Kir subunits are expressed in retinal Muller (glial) cells using immunocytochemistry. (2) By determining the electrophysiological properties and regulation by ATP, spermine and pH of homomeric and heteromeric channels expressed in HEK cells. (3) The electrophysiological properties and regulation of Kir channels and dissociated Muller cells will be compared with the expressed channels and will be examined under different metabolic conditions. The results of these studies will provide insight into normal Kir channel function as well as Kir channel regulation during anoxia and hypoglycemia. when ATP is depleted.